Getting geared?

I'm about to start doing dungeons once I hit Lv.30 in a while so I was wondering how gearing in this game works.
Since each dungeon has a rare and exotic set, is it worth waiting to get exotics at low-level dungeons, or should I just get rares for now?
Also, before starting in the Ascalonian Catacombs, what gear should I get to go in? I'm a guardian btw.

Well, first off let me congratulate you on lv30!
Let me try to explain-
You have 6 pieces of armor, and 6 accessories.
(Head armor, shoulder, chest, gloves, legs, feet) (back item, x2 earring, x2 rings, and an amulet)
You have a few different rarities (qualities) of items, in which the highest tier is Ascended. (Only ascended rings\back items are in game atm). Otherwise items of Exotic rarity are the best you can get.\
Dungeons have 2 modes, story mode and explorable mode, the former gives access to the latter, and the latter gives the same rewards but also dungeon tokens, which, as you said, you can exchange.
In lower levels you would usually level up too fast for it to make sense to use those tokens for level appropriate armor, because when you reach max level you will be able to trade them in for the best gear available.

I'd say pretty much until you are solid about your build and have a few more traits, you shouldn't worry about your gear too much, you can pretty much use whatever you have\like.

Unlike other games, in GW2 the dungeon gear is meaningless. You do not need to ever get a single piece of gear from any dungeon. You only get it for the looks if you like it.

To do story mode dungeons, have a relevant weapon of every type you can use so you can be flexible in what you do. Full green level relevant gear would be a really good idea.

Explorable modes are much harder. Being higher level helps because down-scaling is not 100%.
You are probably going to find that most explorable mode groups want level 80s preferably geared in exotic gear. You might get away with starting at full yellow gear in some groups or guild groups.

Dungeon tokens are exclusive to each dungeon. If you want a dungeon armor set you will do each of the different branches of that dungeon daily for a couple weeks.

From what I remember the rare armor is the same level as the explorable paths. And the exotic version are just as good or bad as any other exotic gear. Since there's no ascended amor yet, exotic armor is the best stat wise. If you get these exotics through karma, crafting, dungeon tokens or the black lion trading company doesn't matter - only that they are supporting your build.

Oh ok. Right now, my build focuses on toughness and vitality, but the guide says it changes later on.

Then I'd get green (masterwork) gear that gives you these stats and an offensive one. That can be condition damage, precision or critical damage depending on what your goal is. At least that's my personal opinion. Green is not as expensive as rare and works well enough for open world PvE and WvW until you hit 80.

Quite frankly, you can sleepwalk your way to 80 in equivalent level green gear and wouldn't be handicapped except in explorable modes and WvW.

Save your tokens and karma for 80 exotics. Most people should be fairly close to a piece of karma gear upon reaching 80, and if you've been hitting the same dungeon explorable, you might be able to get a few dungeon pieces straight off the bat.

The main good thing about dungeon exotics is that there are 3 sets to choose from in terms of stats. You'll very likely want to replace the Runes anyway. Getting karma and dungeon exotics mean you spend gold on Runes and Jewelry instead.